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If you want repairs backed by Apple it's probably better to use one of these locations: 

These are the places that Apple lists for phones.  To change the product to be repaired such as Mac, Apple TV, etc, you will need to change the product to another one in the box.  For Hanoi or other cities do the same.

Thanks, your blood is worth botteling

As far as storage space, you cannot upgrade the storage space anywhere unless you change the phone. I recommend a sturdy case so you don't crack your phone. Not sure what the selection is like over in VN but plenty on Amazon over here. I especially like the Otterbox cases.

OOPS,

I guess anything is possible like turning your combustion engine car into an electric car. The question is how are you going to get the parts and equipment to make the change and will it be cheaper to sell what you have to get what you want or try something you saw on YouTube and it doesn't work out like the video?

pretty cheap

I ordered it and they said delivery by  3 October, when i checked the progress they changed to 25 December

    I ordered it ....
   

    -@MarkinNam

So it appears you didn't like qnbui's excellent advice.  One nice thing in VN is that there is a good market for used phones.   In the US they are virtually worthless because the carriers are giving out new ones for free.  Cheap yes but please let us know how the repairs go after Christmas.  Good luck.